The present invention relates to the field of relays, and in particular to a low power consumption relay which includes an actuator formed from piezoelectric elements which undergo displacement according to applied voltage.
Relays whose motive power is provided by electromagnetic coils are per se well known. Recently, the concept of using a piezoelectric element as the actuating member for a relay, instead of an electromagnetic coil, has been mooted. However, the following problem related to hysteresis has arisen. In FIG. 8 of the accompanying drawings, there is shown a typical characteristic of such a piezoelectric element with regard to the strain produced in it by applied voltage. If such an element is used as the actuator for a relay, then in a typical operational cycle, from the point O where no voltage is applied thereto, first an initial actuating voltage V1 is applied thereto to bring the operational condition and the strain to be as indicated by the point P1, and then the applied voltage is raised to the voltage V2 to bring the operational condition to the point P2. This causes the actuator to be displaced by the amount S2 from its original position, which is effective for switching over the contacts of the relay to their actuated position, as desired. However, if the application of this voltage V2 is continued for a considerable time, which is a condition that must be reckoned with, then the displacement of the piezoelectric element will increase from the amount S2 to a larger amount S4, so as to bring the operational condition to the point P4 in the diagram. If thereafter the applied voltage is reduced back to zero, the system transits to the operational condition indicated by the point P5, and it may well be the case that the displacement S5 corresponding to this zero voltage point P5 does not reach as low as the amount of displacement S3 at the point P3 at which the contacts of the relay are restored to their original or restored condition. In such a case, the contacts of the relay are not properly switched in the restoring direction, and the relay does not function properly. Thus, this hysteresis effect can interfere with proper operation of the relay.